The present invention relates to waveform formats for use in radar applications.
Previous stretch and chirp waveform formats and receiver and exciter architectures generally used the straightforward technique of generating a transmit waveform with a frequency sweep that is centered on the operating channel frequency. For stretch waveform processing, the first LO is similarly modulated but with an extended sweep time to cover the selected range swath and timed to coincide with the return signal from that swath. For chirp waveform processing, the first LO is unmodulated during the return signal processing time. A single or double conversion receiver converts the return signal to a desired final IF by mixing it with one or two LO signals at frequencies that are suitably offset from the transmit frequency and first IF. This approach requires dedicated circuity to develop the frequency offset for the first LO and to generate the second LO and also requires additional receiver processing functions for the double conversion approach. Additionally, if a frequency up or down converter is used to derive the transmit signal from the first LO or visa versa, an additional opportunity is introduced for the introduction of waveform distortions.
Typical implementations of receivers for reconnaissance radar applications that use stretch or chirp waveforms consist of a double conversion architecture to convert the microwave input signal through two IFs prior to synchronous I/Q signal detection. The double conversion process allows RD and IF filtering to provide excellent image signal and image noise rejection at each mixer but requires a substantial amount of receiver and exciter hardware to provide the two frequency conversions, generate the microwave first LO and the typically L-band or UHF second LO, and provide separate filtering in each of the two IFs. In many reconnaissance applications, however, 20 dB of image rejection is sufficient. That degree of rejection can be provided by an image rejection mixer without resorting to multiple IFs.
The present invention makes use of a single conversion receiver architecture and particular formats for the chirp and stretch waveforms to allow hardware simplification even beyond that of a conventional signal conversion receiver. The technique produces a non-zero receiver IF to allow I/Q signal detection at a reasonably low frequency but without the need for additional hardware to generate separate LO operating frequencies that are offset from the transmit frequency. Hardware space savings over conventional approaches are significant.
An additional benefit of the invention is that waveform distortion producing elements in the transmitter drive output path are eliminated. A typical current approach uses an upconverter, comprised of a microwave mixer and output bandpass filter, to derive the transmitter drive signal from a sample of the first LO. The filter functions to select the desired mixer output sideband but also introduces phase distortion on the transmit waveform which can degrade radar imaging quality.